net: 8021q: resolve forwarding path for vlan devices

Add .ndo_fill_forward_path for vlan devices.

For instance, assuming the following topology:

                   IP forwarding
                  /             \
            eth0.100             eth0
            |
            eth0
            .
            .
            .
           ethX
     ab:cd:ef:ab:cd:ef

For packets going through IP forwarding to eth0.100 whose destination
MAC address is ab:cd:ef:ab:cd:ef, dev_fill_forward_path() provides the
following path:

        eth0.100 -> eth0
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -850,11 +850,18 @@ typedef u16 (*select_queue_fallback_t)(struct net_device *dev,
 
 enum net_device_path_type {
 	DEV_PATH_ETHERNET = 0,
+	DEV_PATH_VLAN,
 };
 
 struct net_device_path {
 	enum net_device_path_type	type;
 	const struct net_device		*dev;
+	union {
+		struct {
+			u16		id;
+			__be16		proto;
+		} encap;
+	};
 };
 
 #define NET_DEVICE_PATH_STACK_MAX	5

net/8021q/vlan_dev.c

@@ -776,6 +776,20 @@ static int vlan_dev_get_iflink(const struct net_device *dev)
 	return real_dev->ifindex;
 }
 
+static int vlan_dev_fill_forward_path(struct net_device_path_ctx *ctx,
+				      struct net_device_path *path)
+{
+	struct vlan_dev_priv *vlan = vlan_dev_priv(ctx->dev);
+
+	path->type = DEV_PATH_VLAN;
+	path->encap.id = vlan->vlan_id;
+	path->encap.proto = vlan->vlan_proto;
+	path->dev = ctx->dev;
+	ctx->dev = vlan->real_dev;
+
+	return 0;
+}
+
 static const struct ethtool_ops vlan_ethtool_ops = {
 	.get_link_ksettings	= vlan_ethtool_get_link_ksettings,
 	.get_drvinfo	        = vlan_ethtool_get_drvinfo,
@@ -814,6 +828,7 @@ static const struct net_device_ops vlan_netdev_ops = {
 #endif
 	.ndo_fix_features	= vlan_dev_fix_features,
 	.ndo_get_iflink		= vlan_dev_get_iflink,
+	.ndo_fill_forward_path	= vlan_dev_fill_forward_path,
 };
 
 static void vlan_dev_free(struct net_device *dev)